It is well known to separate particles, such as dirt and dust particles from a fluid flow using mechanical filters, such as foam filters, cyclonic separators and electrostatic separators where dust particles are charged and then attracted to another oppositely charged surface for collection.
Known cyclonic separating apparatus include those used in vacuum cleaners. Such cyclonic separating apparatus are known to comprise a low efficiency cyclone for separating relatively large particles and a high efficiency cyclone located downstream of the low efficiency cyclone for separating the fine particles which remain entrained within the airflow (see, for example, EP 0 042 723B).
Known electrostatic filters include frictional electrostatic filters and electret medium filters. Examples of such filters are described in EP0815788, U.S. Pat. No. 7,179,314 and U.S. Pat. No. 6,482,252.
Such electrostatic filters are relatively cheap to produce but suffer from the disadvantage that their charge dissipates over time resulting in a reduction of their electrostatic properties. This in turn reduces the amount of dust the electrostatic filter can collect which may shorten the life of both the electrostatic filter itself and any further downstream filters.
Known electrostatic filters also include filters where dust particles in an airstream are charged in some way and then passed over or around a charged collector electrode for collection. An example of such a filter is described in JP2007296305 where dust particles in an airstream are charged as they pass a “corona discharge” wire and are then trapped on a conductive filter medium located downstream of the corona discharge wire. A disadvantage with this arrangement is that they are relatively inefficient, are made from relatively expensive materials and the collector electrodes require constant maintenance in order to keep them free of collected dust. Once the collector electrodes are coated in a layer of dust they are much less efficient.
Another example is shown in GB2418163 where the dust particles in an airstream are charged as they pass a corona discharge wire located inside a cyclone. The charged dust particles are then trapped on the walls of the cyclone which are coated in a conductive paint. While this arrangement is compact it suffers from the disadvantage that dust collects on the inside of the cyclones. Not only does this require constant and difficult maintenance removing dust from the walls of the cyclone, but also any dust trapped inside the cyclone will interfere with the cyclonic airflow decreasing the separation efficiency of the cyclone itself.
Another example is shown in U.S. Pat. No. 5,593,476 where a filter medium is placed between two permeable electrodes and the airflow is arranged to pass through the electrodes and through the filter media.
It is desirable for the efficiency of an electrostatic filter to be as high as possible (i.e. to separate as high a proportion as possible of very fine dust particles from the airstream), while maintaining a reasonable working life. It is also desirable that the electrostatic filter does not cause too much of a pressure drop across it.
An electrostatic filter which could provide high efficiency along with a long working life would therefore be desirable. In certain applications, for example in domestic vacuum cleaner applications, it is desirable for the appliance to be made as compact as possible without compromising the performance of the appliance. An electrostatic filter which was simpler in construction allowing easy packaging into an appliance would therefore also be desirable.